Field of the Invention
The present invention relates to a processing method of substrate in which in forming a trench on the substrate by etching, a side wall surrounding the trench can be precisely formed, and a manufacturing method of liquid ejection head.
Description of the Related Art
In performing processing of forming a trench, such as a deep hole or groove, on a silicon substrate made of silicon, the Bosch process is occasionally employed as a method for the processing. In the Bosch process, steps of etching a substrate, forming a passivation layer, which protects the substrate, on a portion where the etching was performed, and removing the passivation layer on a portion to be excavated are sequentially repeated. In performing the processing, in a state in which the passivation layer is formed in a lateral direction on a side wall surrounding the trench, the trench is excavated. Since the trench is excavated in a state in which the side wall is protected by the passivation layer, in performing etching, local expansion of the trench in a direction crossing a depth direction is suppressed to allow the trench to be appropriately excavated in the depth direction.
However, in a case of performing etching for forming a deep trench having a high aspect ratio, the number of times of repeating the above steps increases. In this case, for further excavation of the trench, the step of removing the passivation layer on a bottom portion of the trench is also repeated. In the step of removing the passivation layer on the bottom portion of the trench, irradiation of an SF6 (sulfur hexafluoride) gas, for example, to the bottom portion of the trench is performed. The bottom portion of the trench is exposed to the gas such as SF6, and whereby the passivation layer covering a position at which the trench is excavated is removed.
In a case of removing the passivation layer by using the gas such as SF6, ion tracks are not always ideally perpendicular to the substrate. Since ions present in plasma have speeds in all directions, in irradiating the substrate with the ions, the ions have a speed also in a component in a horizontal direction crossing the depth direction of the trench. The ions have the speed remaining in the horizontal direction, and thus ions colliding with the side wall surrounding the trench exist. Accordingly, the ions collide with the side wall to scrape off the side wall, and as a result, the side wall may possibly be deformed.
In a shallow portion of the trench, each time the step of removing the passivation layer is repeated, collision between the side wall of the trench and the ions moving in the horizontal direction occurs. Therefore, in the shallow portion of the trench, as the number of times of performing the step of removing the passivation layer increases, the number of times of collision between the side wall and the ions increases.
Japanese Patent Laid-Open No. 2000-299310 discloses a method in which, for the purpose of reducing deformation of a side wall, an oxide film (SiO2) obtained by oxidizing a silicon substrate is formed on the side wall of a trench. Since the oxide film of silicon is highly resistant to damage at the time of collision with ions, forming the trench in a state in which the oxide film is formed on the side wall, by means of the Bosch process, can suppress deformation of the side wall. In Japanese Patent Laid-Open No. 2000-299310, by generating oxygen plasma in a chamber in which the substrate is disposed to oxidize the silicon substrate, the oxide film is formed on the side wall of the trench.